The present invention relates to a method for controlling the distribution of different transmission frequencies in a radar station of the high resolution type, in which the transmission frequency of the transmitted pulses is changed between the pulses.
The range resolution which is possible to attain with a radar station is in principle determined by the bandwidth of the transmitted radar pulse. For a good range resolution it is desirable to make the transmitted pulse very short which results in a wide bandwidth. As the radar transmitter's maximum output is limited, short transmission pulses will result in a reduction of the transmitted amount of energy with, among other things, a reduced range as a consequence. The possibility to improve the range resolution by shortening transmission pulses is therefore limited.
The development of high resolution radar stations has therefore gone towards a technique in which special signal processing improves the resolution. This type of radar system often uses pulse compression in some form to achieve a good range resolution. For example, frequency coding of the transmitted pulse can be used by sweeping the transmission frequency during the time interval of the pulse. This method, however, has the disadvantage that it requires a wide bandwidth in large parts of the radar system, which is often difficult and/or costly to achieve.
A method which requires a narrower momentary bandwidth is previously known from U.S. Pat. No. 4,851,848. In this patent a radar system is described in which the conventional pulse compression is replaced with a system where the transmission frequency for a certain number of transmitted pulses is changed step by step from pulse to pulse within a certain frequency range. After the transmission of such a sequence ("burst") of pulses with different frequencies, the sequence is repeated. The signals received from a number of sequences are stored together and, by using among other things Fourier transformation, a very good range resolution can be obtained without the need for a wide momentary bandwidth in the radar.
A similar example of a radar system using frequency stepping from pulse to pulse is also described in the book "High Resolution Radar" by Donald R Wehner, Artech House, 1987, chapter 5.
In every radar system there is also the requirement to be able to separate a useful signal from interfering signals. What is a useful respective interfering signal depends on the application. If, for example, the useful signal stems from an airplane body, then ground clutter, engine modulation etc. are interfering signals. To filter out the interfering signals it is possible to take advantage of the fact that they have different Doppler frequencies. The filtration of Doppler frequencies necessitates a number of pulses using one and the same transmission frequency where the filtration performance characteristic is set by the number of pulses and their repetition frequency.
Common to all known systems is that all the different frequencies which are used are part of each sequence. When the frequency is Stepped between a large number of frequencies it will take a long time to complete one sequence before the next sequence can be started. Accordingly, it will take a long time for two pulses with the same frequency to repeat. This time interval is called FRI (Frequency Repetition Interval) and its repetition frequency FRF=1/FRI. This means a relatively low FRF when FRF=PRF/(number of frequencies) where PRF denotes the pulse repetition frequency of the radar system. It is desirable, especially in airborne applications, to have a higher FRF to obtain the necessary performance characteristic for the Doppler filtration. One way of changing the FRF is to increase the sweep speed, which increases the PRF which determines the time between two transmitted pulses. This parameter cannot, for different reasons, be set too high or be chosen freely.
Another parameter of great interest, especially for moving targets, is the total measuring time. The measuring time is to be understood as the time for transmitting the required number of pulses needed for the Doppler filtration, on all the transmitted frequencies.
In the case with frequency stepping, the measuring time=PRI*(total number of necessary pulses) and FRF=PRF/(number of frequencies).
In another case it is conceivable to transmit a number of pulses on one frequency, change the frequency and transmit a number of pulses on this new frequency and so on until all frequencies have been cycled through. In this case the measuring time will be=PRI*(total number of pulses) and FRF=PRF.
From these two cases it is evident that, when the FRF is fixed, then the first case results in a high PRF and a short measuring time while the second case results in a low PRF and a long measuring time, which implies that it is not possible to combine a "just right" PRF with a short measuring time.